Rate of Phosphate Release after Photoliberation of Adenosine 5′-Triphosphate in Slow and Fast Skeletal Muscle Fibers

Abstract

Inorganic phosphate (P i) release was determined by means of a fluorescent P i-probe in single permeabilized rabbit soleus and psoas muscle fibers. Measurements of P i release followed photoliberation of ∼1.5 mM ATP by flash photolysis of NPE-caged ATP in the absence and presence of Ca 2+ at 15°C. In the absence of Ca 2+, P i release occurred with a slow rate of 11 ± 3 μM · s −1 ( n = 3) in soleus fibers and 23 ± 1 μM · s −1 ( n = 10) in psoas fibers. At saturating Ca 2+ concentrations (pCa 4.5), photoliberation of ATP was followed by rapid force development. The initial rate of P i release was 0.57 ± 0.05 mM · s −1 in soleus ( n = 13) and 4.7 ± 0.2 mM · s −1 in psoas ( n = 23), corresponding to a rate of P i release per myosin head of 3.8 s −1 in soleus and 31.5 s −1 in psoas. P i release declined at a rate of 0.48 s −1 in soleus and of 5.2 s −1 in psoas. P i release in soleus was slightly faster in the presence of an ATP regenerating system but slower when 0.5 mM ADP was added. The reduction in the rate of P i release results from an initial redistribution of cross-bridges over different states and a subsequent ADP-sensitive slowing of cross-bridge detachment.